Wrist driving mechanism for industrial robot

ABSTRACT

A wrist driving mechanism for an industrial robot has a first base wrist unit (13) supported on the free end of a robot arm (11) and capable of rotating about a first axis (γ). The first base wrist unit (13) is mounted with a second base wrist unit (14) capable of rotating about a second axis (β). The second base wrist unit (14) is mounted with a fore wrist unit (15) capable of rotating about a third axis (α). The robot arm (11) is provided along the longitudinal direction thereof with a first power transmitting unit (16) for transmitting a rotative power to the first base wrist unit (13). A second power transmitting unit (19) for transmitting a rotative power to the second base wrist unit (14) is formed along the robot arm (11) and the first base wrist unit (13). A motor (25) for driving the fore wrist unit (15) for rotation is mounted on the first base wrist unit (13). The driving shaft of the motor (25) is coupled with a first transmission shaft (26) extended along the second axis (β). The first transmission shaft (26) is connected through a gear train (27) to the second transmission shaft (28) extended along the third axis (α) and the second transmission shaft (28) is connected to the fore wrist unit (15). The second transmission shaft (28) is associated with a reduction gear mechanism (31).

DESCRIPTION

1. Technical Field

The present invention relates to a wrist driving mechanism for anindustrial robot, and more specifically, to a wrist driving mechanismfor driving the fore wrist unit of an industrial robot for rotary motionabout three axes.

2. Background Art

There has been proposed a three-axis wrist driving mechanism includingthree power transmitting chains extended in parallel to each other alongthe robot arm to drive the fore wrist of the wrist device of anindustrial robot for rotary motion about three axes. Such a conventionalthree-axis wrist driving mechanism is shown in FIG. 1. As shown in FIG.1, a first base wrist unit 2 is joined to the free end of a robot arm 1so as to be rotatable about an axis γ. A second base wrist unit 3 isjoined to the first base wrist unit 2 so as to be rotatable about anaxis α. A fore wrist unit 4 is joined to the second base wrist unit 3 soas to be rotatable about an axis α. Three sprockets 8, 9, and 10 aredisposed rotatably within the free end of the robot arm 1. The sprockets8, 9, and 10 are interlocked with the first base wrist unit 2, thesecond base wrist unit 3, and the fore wrist unit 4 respectively. Threechains 5, 6, and 7 are extended in parallel to each other within andalong the longitudinal direction of the arm 1. The chains 5, 6, and 7interconnect the sprockets 8, 9, and 10 and three corresponding drivingsprockets, not shown, disposed side by side at the base end of thearm 1. The driving sprockets are connected through reduction gearmechanisms to driving motors, respectively. Ordinarily, functionalequipment, such as a robot hand, a welding gun or a spray gun, ismounted on the fore wrist unit 4.

The rigidities of the output system about the axes α, β and γ of a threeaxis wrist driving mechanism of the above-mentioned constitution aredependent on the spring constants k (k are fixed values) of the chains5, 6, and 7. Particularly, when a torque about the axis β is applied tothe second base wrist unit 3, the first base wrist unit 2 tends to beturned about the axis γ. Therefore, the rigidity of the output systemabout the axis β is dependent on the spring constants of the two chains5 and 6. That is, the total spring constant kβ of the output systemabout the axis β is expressed by an expression: kβ=k/2.

When a torque about the axis α is applied to the fore wrist unit 4, thesecond base wrist unit 3 tends to be turned about the axis β, and hencethe first base wrist unit 2 also tends to be turned about the axis γ.Accordingly, the rigidity of the output system about the axis α isdependent on the spring constants of the three chains 5, 6, and 7.Consequently, the total spring constant kα of the output system aboutthe axis α is expressed by an expression kα=k/3.

Accordingly, for the output system about the axis α, the total springconstant is k/3 and the actual rigidity of the output system isone-third of the inherent rigidity thereof, even if chains of springconstant k are employed. Furthermore, since the spring constant k of achain is in inverse proportion to the length of the chain, the rigidityof the output system about the axis α is reduced with the length of thechains.

In the above-mentioned wrist driving mechanism, a rotative motion of areduced speed controlled by the reduction gear mechanism is transmittedthrough a gear train to the fore wrist unit. Therefore, backlash betweenthe teeth of the gears reduces the rigidity of the output system.

DISCLOSURE OF THE INVENTION

Accordingly, in view of the disadvantages of the above-mentionedthree-axis wrist driving mechanism of the chain-drive type, it is anobject of the present invention to provide a wrist driving mechanism foran industrial robot, capable of enhancing the rigidity of the fore wristunit of the three-axis wrist device about the axes of rotation thereof.

The present invention provides a wrist driving mechanism for anindustrial robot, comprising: a first base wrist unit supported on thefree end of a robot arm so as to be rotatable about a first axisrelative to the robot arm; a second base wrist unit supported on thefirst base wrist unit so as to be rotatable about a second axis relativeto the first base wrist unit; a fore wrist unit supported on the secondbase wrist unit so as to be rotatable about a third axis relative to thesecond base wrist unit; a first power transmitting unit disposed alongthe longitudinal direction of the robot arm to transmit a rotative powerto the first base wrist unit; a second power transmitting unit disposedalong the robot arm and the first base wrist unit to transmit a rotativepower to the second base wrist unit; a motor mounted on the first basewrist unit to drive the fore wrist unit for rotation; a firsttransmission shaft coupled with the driving shaft of the motor andextended along the second axis; a second transmission shaft connected tothe fore wrist unit, interlocked through a gear train with the firsttransmission shaft and extended along the third axis; and a reductiongear mechanism associated with the second transmission shaft.

According to the present invention, the power transmission systeminterlocking the fore wrist unit with the motor mounted on the firstbase wrist unit comprises the first transmission shaft, the gear train,the second transmission shaft and the reduction gear mechanism,therefore, the spring constant of the power transmission systeminterlocking the fore wrist unit with the motor is extremely large ascompared with that of a power transmission system of the chain-drivetype. Particularly, since the reduction gear mechanism is interposedbetween the output side of the gear train for transmitting the drivingforce of the motor and the fore wrist unit, backlash between the teethof the gears of the gear train and the torsion of the transmissionshafts scarcely affect the rigidity of the fore wrist unit. Thus thepresent invention is capable of providing a wrist driving mechanismwhich ensures a fore wrist having a high rigidity.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome more apparent from the following description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a three-axes wrist drivingmechanisms of a conventional industrial robot;

FIG. 2 is a front elevation of a wrist driving mechanism, in a preferredembodiment, according to the present invention, for an industrial robot;

FIG. 3 is a schematic illustration showing the constitution of the wristdriving mechanism of FIG. 2;

FIG. 4 is an enlarged longitudinal sectional view of the essentialportion of the wrist driving mechanism of FIG. 2;

FIG. 5 is a sectional view of the wrist driving mechanism of FIG. 2taken on line V--V of FIG. 4; and

FIG. 6 is a fragmentary perspective view of a partial modification ofthe wrist driving mechanism of FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 2 to 5 show a preferred embodiment of the present invention.Referring to FIGS. 2 and 3, an industrial robot is provided with a wristdevice 12 capable of rotary motion about three axes, mounted on the freeend of a robot arm 11, and the wrist device comprises a first base wristunit 13, a second base wrist unit 14 and a fore wrist unit 15.

The first base wrist unit 13 is supported so as to be rotatable about afirst axis γ relative to the free end of the robot arm 11. The secondbase wrist unit 14 is supported so as to be rotatable about a secondaxis β relative to the first base wrist unit 13. The fore wrist unit 15is supported so as to be rotatable about a third axis α relative to thesecond base wrist unit 14. The axis γ extends perpendicularly to thelongitudinal axis C of the robot arm 11, while the axis α extendsperpendicularly to the axis β.

The industrial robot is provided with a first power transmitting unit 16for transmitting power to the first base wrist unit 13. The unit 16comprises a first driven sprocket 17 rotatable about the axis γ and afirst endless chain 18 as a wrapping connector for transmitting arotative power to the sprocket 17. The sprocket 17 is fixed to thehollow shaft 13a of the first base wrist unit 13 so that the sprocket 17is rotatable within the free end of the robot arm 11. The chain 18 isextended along the longitudinal axis C of the robot arm 11. The unit 16includes a first driving sprocket, not shown, disposed at the base endof the robot arm 11, for driving the chain 18. The first drivingsprocket is connected through a reduction gear mechanism, not shown, toa first driving motor, not shown.

The industrial robot is provided with a second power transmitting unit19 for transmitting power to the second base wrist unit 14. The unit 19comprises a second driven sprocket 20 disposed within the free end ofthe robot arm so as to be rotatable, a second endless chain 21 as awrapping connector for transmitting a rotative power to the sprocket 20,and a pair of bevel gears 22 and 23 for transmitting the rotary motionof the sprocket 20 to the second base wrist unit 14. The input bevelgear 22 is fixed through a shaft 24 extending coaxially with the hollowshaft 13a of the first base wrist unit 13 to the sprocket 20 so as to berotatable about the axis γ relative to the first base wrist unit I3. Theoutput bevel gear 23 is fixed to the hollow shaft 14a of the second basewrist unit 14 for rotation about the axis β relative to the first basewrist unit 13. The second endless chain 21 is extended along thelongitudinal axis C of the robot arm 11 in parallel to the first endlesschain 18. The unit 19 further includes a second driving sprocket, notshown, for driving the chain 21, disposed at the base end of the robotarm 11. The second driving sprocket is connected through a reductiongear mechanism, not shown, to a second driving motor, not shown.

The first base wrist unit 13 is provided with a motor 25 to drive thefore wrist unit 15 for rotation. The driving shaft 25a of the motor 25is coupled with a first transmission shaft 26 extended through thehollow shaft 14a of the second base wrist unit 14 along the axis β. Thefirst transmission shaft 26 is interlocked with a second transmissionshaft 28 by a gear train 27. The second transmission shaft 28 isextended along the axis α and is connected to the fore wrist unit 15.The gear train 27 consists of a pair of bevel gears 29 and 30.

A reduction gear mechanism 31 is interposed in the second transmissionshaft 28. In this embodiment, the second transmission shaft 28 has aninput shaft 28a connected to the output bevel gear 30 of the gear train27 and an output shaft 28b connected to the fore wrist unit 15.Referring to FIGS. 4 and 5, the reduction gear mechanism 31 has anelliptic inner member 32 fixed to the input shaft 28a. An elastic ringgear 33 having teeth formed along the outer circumference thereof ismounted through a bearing 34 on the circumference of the inner member 32so as to be movable in the circumferential direction thereof. A firstouter member 35 having teeth which engage the teeth of the ring gear 33is fixed to the inner surface of the second base wrist unit 14. A secondouter member 36 internally provided with teeth which engage the teeth ofthe ring gear 33 is fixed to the output shaft 28b. The reduction gearmechanism of this constitution is advantageous, as compared with anordinary planetary gear mechanism, in that the play of the output siderelative to the input side attributable to the backlash can beremarkably reduced. When the reduction ratio of this reduction gearmechanism 31 is desired to be 1/100, the numbers of teeth of the ringgear 33, the first outer member 35 and the second outer member 36 are,for example, 100, 101, and 101, respectively.

In this wrist driving mechanism according to the present invention,driving the first driven sprocket 17 by the first chain 18 causes thefirst base wrist unit 13 to rotate together with the first drivensprocket 17 about the axis γ, and driving the second driven sprocket 20by the second chain 21 causes the input bevel gear 22 to rotate aboutthe axis γ, so that the output bevel gear 23 engaging the input bevelgear 22 rotates together with the second base wrist unit 14 about theaxis β.

On the other hand, when the motor 25 is actuated and the driving shaft25a of the motor 25 rotates, the input bevel gear 29 rotates about theaxis β, and thereby the output bevel gear 30 rotates together with theinput shaft 28a about the axis α. The rotary motion of the input shaft28a is transmitted through the reduction gear mechanism 31 to the outputshaft 28b, in which the revolving speed of the input shaft 28a isreduced to a lower revolving speed of the output shaft 28b so that theoutput shaft 28a rotates together with the fore wrist unit 15 about theaxis α.

A torque applied to the second base wrist unit 14 tends to turn thefirst base wrist unit 13. Therefore, the total spring constant kβ of theoutput system about the axis β, as viewed from the output side, isexpressed by an expression: kβ=k/2, where k is the spring constant ofthe first chain 18 and the second chain 21.

On the other hand, in the wrist driving mechanism according to thepresent invention, the power transmitting system interlocking the motor25 with the fore wrist unit 15 comprises the first transmission shaft26, the gear train 27, the second transmission shaft 28, and thereduction gear mechanism 31. Accordingly, the spring constant of thepower transmitting system interconnecting the motor and the fore wristunit is extremely large as compared with that of the conventionalchaindriven driving system. Particularly, since the reduction gearmechanism is interposed between the output side of the gear train 27 fortransmitting the driving force of the motor 25 and the fore wrist unit15, backlash in the gear train 27 and the torsion of the firsttransmission shaft 26 interconnecting the motor 25 and the gear train 27scarcely affect the rigidity of the fore wrist unit 15. Consequently,the rigidity of the fore wrist unit 15 is enhanced.

This embodiment employs the endless chains 18 and 21 as wrappingconnectors for transmitting rotative power to the first driven sprocket17 and to the second driven sprocket 20, respectively. However, thewrapping connector of the first power transmitting unit 16 may comprisea chain 18a of a predetermined length engaging the first driven sprocket17, a chain 18b of a predetermined length engaging the first drivingsprocket 37, and two parallel rods 18c and 18d interconnecting therespective opposite ends of the chains 18a, and 18b as shown in FIG. 6.Preferably, tighteners 18e and 18f are interposed between the rods 18cand 18d and either the chain 18a or the chain 18b. The first drivingsprocket 37 is connected through a reduction gear mechanism 38 to thedriving motor 39. Although not illustrated, the wrapping connector ofthe second power transmitting unit 19 may be constructed similarly tothat of the first power transmitting unit 16. Since the length of thechains of such a wrapping connector employing rods can be shortened, thespring constant of the wrapping connector can be enhanced, whereby therespective rigidities of the output systems of the wrist mechanism areenhanced. When wrapping connectors of such a constitution are employed,it is desirable to reduce the lengths of the chains to the minimumpossible lengths necessary for ensuring the action of the first andsecond base wrist units in the respective working ranges. Such aconstitution restricts the reduction of the rigidity attributable to theelasticity of the wrapping connectors to a least extent.

The reduction gear mechanism 31 may be a reduction gear of another typesuch as a cycloreduction gear.

CAPABILITY OF EXPLOITATION IN INDUSTRY

A wrist driving mechanism for an industrial robot, according to thepresent invention is effectively applicable to articulated industrialrobots for various work such as assembling, welding, and painting.

We claim:
 1. A wrist driving mechanism for an industrial robot,comprising:an elongated robot arm having a free end; a first base wristunit extending laterally from one side of said free end of said robotarm and rotatably supported about a first axis which intersects alongitudinal axis of said robot arm at a right angle; a second basewrist unit rotatably supported on said first base wrist unit about asecond axis which intersects said first axis at a right angle; a forewrist unit rotatably supported on said second base wrist unit about athird axis which intersects said second axis at a right angle, said forewrist unit being adapted to enable functional equipment to be mountedthereon; a first power transmitting unit for transmitting a rotativepower of a first drive motor provided on said robot arm to said firstwrist base unit; a second power transmitting unit for transmitting arotative power of a second drive motor provided on said robot arm tosaid second base wrist unit; a third drive and rotating said fore wristunit about said third axis, said third drive motor having a drive shaftarranged coaxially with said second axis; first and second bevel gearsmeshing with each other within said second base wrist unit, and firstbevel gear, coaxial with said second axis, is fixedly connected to saiddrive shaft of said third drive motor through a first transmission shaftextending coaixally with said second axis, and said second bevel gear iscoaxial with said third axis; and a reduction gear unit supported onsaid fore wrist unit and having input and output shafts which extendcoaxially with said third axis, said input shaft being fixed to saidsecond bevel gear and said output shaft being fixed to said fore wristunit.
 2. The wrist driving mechanism of claim 1 wherein said reductiongear unit further comprises:an inner member fixed to said input shaftand having an elliptical outer circumferential surface; an elastic ringgear movably supported on said elliptical outer circumferential surfaceof said inner member and having outer teeth formed along the outercircumference thereof; a first outer member fixed to said second basewrist unit and having first inner teeth formed along the innercircumference thereof so as to mesh with said outer teeth of said ringgear; and a second outer member fixed to said output shaft and havingsecond inner teeth formed along the inner circumference thereof so as tomesh with said outer teeth of said ring gear.
 3. The wrist drivingmechanism of claim 1 wherein,said first power transmitting unit includesa first sprocket rotatably supported on said robot arm about said firstaxis and fixed to said first base wrist unit, and a first wrappingconnector engaged with said first sprocket to transmit rotation of saiddrive shaft of said first drive motor, and said second powertransmitting unit includes a second sprocket rotatably supported on saidrobot arm about said first axis, and a second wrapping connector engagedwith said second sprocket to transmit rotation of said drive shaft ofsaid second drive motor, a third and fourth bevel gears meshing witheach other within said first base wrist unit, said third bevel gearbeing arranged about said first axis and fixedly connected to saidsecond sprocket through a second transmission shaft passing through saidfirst sprocket, said fourth bevel gear being arranged about said secondaxis and fixed to said second base wrist unit, said first transmissionshaft passing through said fourth bevel gear.
 4. The wrist dirvingmechanism of claim 3, wherein said first wrapping connector includes apair of chains and a pair of parallel rods connecting opposite ends ofone of said chains to opposite ends of the other said chain, and therespective lengths of said chains of said first wrapping connector are aminimum possible length necessary for ensuring action of said first basewrist unit in said working ranges.
 5. The wrist driving mechanism ofclaim 3, wherein said second wrapping connector includes a pair ofchains and a pair of parallel rods connecting opposite ends of one ofsaid chains to opposite ends of the other said chain, and the respectivelengths of said chains of said second wrapping connector are the minimumpossible length necessary for ensuring action of said second base wristunit in said working ranges.